Reduce Ice-generated C++ warnings

Rather than simply listing (and ignoring) all the warnings
that come with ZeroC-generated code we now suppress their
output at the cmake level.

In addition, using `cmake -Dfatal-warnings=true .`
will make all warnings an error which forces cleaning
up these issues.

/cc @rleigh-dundee @sbesson -- suggestions on better header names welcome.
Suggestion: Surround the pragmas with:

```
#ifndef OMEROCPP_ICE_WARN
  <pragmas here>
  #define OMEROCPP_ICE_WARN
#else
#  error "OMEROCPP_ICE_WARN already defined; missing diagnostic pop"
#endif
```